Insect behaviors driven by chemosensation, such as host-seeking, feeding, and mating, have a major impact on human activities. Insects contribute to the spread of a variety of infectious diseases through feeding on human hosts, and they also cause a great deal of agricultural loss through feeding on crops. Insects locate their human or plant hosts using olfactory cues, and they decide whether or not to feed based on gustatory cues found on the host. Bitter compounds are of particular interest because they are behaviorally aversive and can act as feeding deterrents. Although significant progress has been made in understanding insect odor coding at the periphery, much less is known about taste coding, particularly for the sensing of bitter compounds. In contrast to olfactory receptors, the extensive co-expression of bitter receptors in taste neurons makes determination of individual receptor response profiles exceedingly difficult. To address this problem, I have devised an in vivo functional expression system in Drosophila, the decoder bitter neuron system, that allows me to express an individual bitter receptor of the gustatory receptor (Gr) family and record the responses it confers to a wide array of bitter compounds. [I have chosen Drosophila as the model organism because of the wealth of unique genetic and molecular tools available that allowed me to design and develop this system.] I will use this decoder bitter neuron system to analyze different aspects of bitter Gr function, such as tuning breadth and sensitivity, as well as investigate possible interactions of co-expressed Grs. [I will also conduct a pilot analysis of a mosquito predicted bitter Gr, expressed in my system.] This may lead to a greater understanding of how insects, in general, encode bitter tastant identity, which could be applied to insect pests and aid in developing improved feeding deterrents to fight against agricultural crop damage and the spread of infectious disease.